The Single<'TSource>(IEnumerable<'TSource>, Func<'TSource, Boolean>) method throws an exception if the input sequence contains no matching element. To instead return null when no matching element is found, use SingleOrDefault<'TSource>.

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The following code example demonstrates that Single<'TSource>(IEnumerable<'TSource>, Func<'TSource, Boolean>) throws an exception when the sequence does not contain exactly one element that satisfies the condition.